Fergie will be 71 on New Year's Eve but shows no sign of slowing down.
And Becks, who last night said goodbye to LA Galaxy after six seasons in America, believes it is the fire in the Scot's belly which keeps the United boss going in his quest for more silverware. He said: "You can see the aggression in him still.
"It scares me when I watch him on telly. The aggression will never leave him as a manager.
"It's one of the reasons why he is respected like he is, but he has got that tender side as well - not always, but he does have it.
"Looking at him on the sidelines I think he can go on forever.
"It's impossible to say that he's ever going to stop.
"He continues to defy people and continues to defy an age limit in a manager because he's the best, it's as simple as that.
"You hear from so many players who have been in his teams about the respect they have for him as a person and as a manager."